Re: [PATCH v7 3/7] exec: Move path_noexec() check earlier

Eric W. Biederman
2020-08-11T19:40:25Z

Mickaël Salaün <mic@digikod.net> writes:

> From: Kees Cook <keescook@chromium.org>
>
> The path_noexec() check, like the regular file check, was happening too
> late, letting LSMs see impossible execve()s. Check it earlier as well
> in may_open() and collect the redundant fs/exec.c path_noexec() test
> under the same robustness comment as the S_ISREG() check.
>
> My notes on the call path, and related arguments, checks, etc:

A big question arises, that I think someone already asked.

Why perform this test in may_open directly instead of moving
it into inode_permission. That way the code can be shared with
faccessat, and any other code path that wants it?

That would look to provide a more maintainable kernel.

Eric

> do_open_execat()
> struct open_flags open_exec_flags = {
> .open_flag = O_LARGEFILE | O_RDONLY | __FMODE_EXEC,
> .acc_mode = MAY_EXEC,
> ...
> do_filp_open(dfd, filename, open_flags)
> path_openat(nameidata, open_flags, flags)
> file = alloc_empty_file(open_flags, current_cred());
> do_open(nameidata, file, open_flags)
> may_open(path, acc_mode, open_flag)
> /* new location of MAY_EXEC vs path_noexec() test */
> inode_permission(inode, MAY_OPEN | acc_mode)
> security_inode_permission(inode, acc_mode)
> vfs_open(path, file)
> do_dentry_open(file, path->dentry->d_inode, open)
> security_file_open(f)
> open()
> /* old location of path_noexec() test */
>
> Signed-off-by: Mickaël Salaün <mic@digikod.net>
> Signed-off-by: Kees Cook <keescook@chromium.org>
> Link: https://lore.kernel.org/r/20200605160013.3954297-4-keescook@chromium.org
> ---
> fs/exec.c | 12 ++++--------
> fs/namei.c | 4 ++++
> 2 files changed, 8 insertions(+), 8 deletions(-)
>
> diff --git a/fs/exec.c b/fs/exec.c
> index bdc6a6eb5dce..4eea20c27b01 100644
> --- a/fs/exec.c
> +++ b/fs/exec.c
> @@ -147,10 +147,8 @@ SYSCALL_DEFINE1(uselib, const char __user *, library)
> * and check again at the very end too.
> */
> error = -EACCES;
> - if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> - goto exit;
> -
> - if (path_noexec(&file->f_path))
> + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> + path_noexec(&file->f_path)))
> goto exit;
>
> fsnotify_open(file);
> @@ -897,10 +895,8 @@ static struct file *do_open_execat(int fd, struct filename *name, int flags)
> * and check again at the very end too.
> */
> err = -EACCES;
> - if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode)))
> - goto exit;
> -
> - if (path_noexec(&file->f_path))
> + if (WARN_ON_ONCE(!S_ISREG(file_inode(file)->i_mode) ||
> + path_noexec(&file->f_path)))
> goto exit;
>
> err = deny_write_access(file);
> diff --git a/fs/namei.c b/fs/namei.c
> index a559ad943970..ddc9b25540fe 100644
> --- a/fs/namei.c
> +++ b/fs/namei.c
> @@ -2863,6 +2863,10 @@ static int may_open(const struct path *path, int acc_mode, int flag)
> return -EACCES;
> flag &= ~O_TRUNC;
> break;
> + case S_IFREG:
> + if ((acc_mode & MAY_EXEC) && path_noexec(path))
> + return -EACCES;
> + break;
> }
>
> error = inode_permission(inode, MAY_OPEN | acc_mode);